The average speed in the third second is 9 m / s. find (1) the acceleration of the uniform deceleration of the car (2) the displacement of the car in 10 s

The average speed in the third second is 9 m / s. find (1) the acceleration of the uniform deceleration of the car (2) the displacement of the car in 10 s

(1) According to the meaning of the question, the car moves in a straight line at a constant speed in the first two seconds, and starts to move in a straight line at a constant deceleration in the third second, and the average speed in the third second is 9 m / s, that is, the displacement in the third second is 9 m, then the acceleration of the car moving in a straight line at a constant deceleration is a, and the deceleration time is t = 1 s. according to the displacement time relationship, a = 2 × 9 − 2 × 10 × 11 = - 2 m / S2 (2) according to v = v0 + at, the car stops T = V − v0a = 0 − 10 − 2 = 5S, i.e. the car stops moving 5 seconds after the engine is turned off, so the displacement of the car in 10 seconds is the displacement of uniform linear motion in the first 2 seconds plus the displacement of uniform deceleration linear motion in 5 seconds. That is, x = 10 × 2 + [10 × 5 + 12 × (− 2) × 25] = 45m